1. Field of the Invention
The present invention relates to an image forming apparatus wherein a toner layer is formed on the surface of a toner carrying member and is transported, and the latent image formed on the surface of the image carrier is developed with toner. In particular, the present invention relates to a deposited-toner measuring apparatus and an image forming apparatus for detecting the toner deposited on the toner carrying member when a toner layer is formed.
2. Description of the Background Art
There is extensively used an image forming apparatus using an electrophotographic process in which an electrostatic latent image is formed on a photo conductor (image carrier) and is developed with toner, and the toner image is transferred onto paper or another recording medium to be fixed in position.
As the development methods for developing an electrostatic latent image using a dry type developer includes a one-component development method using toner alone and a two-component development method using both toner and carrier are conventionally known.
There is also a development method capable of ensuring high image quality on the same level as that obtained by the one-component development method, as well as providing a long service life as the two-component development method. This is disclosed as a so-called hybrid development method in which a two-component developer is carried on a developer carrying member and only toner is supplied from the two-component developer to the toner carrying member to per form development (For example, unexamined Japanese Patent Application Publication No. H05 (1993)-150636).
In the hybrid development method, a bias voltage is applied to supply toner from the developer to the toner carrying member. The toner layer formed on this toner carrying member is used to develop the latent image on the image carrier arranged facing the toner carrying member.
In the one-component development method or the hybrid development method, if there is a change in the toner amount (deposited-toner amount) of the toner layer formed on the toner carrying member, the state of image formation is changed, resulting in a change in image density which affects the image quality.
Thus, to get stable image quality, it is important to ensure that the amount of toner deposition on the toner layer formed on the toner carrying member is kept at a constant level. For that purpose, at the time when forming a toner layer on the toner carrying member, it is required to accurately detect the amount of the deposited toner, which fluctuates depending on the printing environment, the total number of printed sheets and/or the number of continuous printing sheets.
As one of the methods for detecting the amount of toner deposited on the toner layer formed on the toner carrying member, an optical detection method is commonly known (Unexamined Japanese Patent Application Publication No. 2008-176236). This method uses an LED or LD as a light emitting section. The emitted light is applied to the toner layer, and the reflected light is detected by a light receiving element such as a photoelectric conversion element, and the absolute amount of the toner layer is obtained from the intensity of this reflected light.
In another method, the electric charge amount of toner supplied from the developer carrying member to the toner carrying member is obtained by analyzing the current flowing through a closed loop circuit made up of a toner carrying member, a developer carrying member and a bias power supply connected therebetween (e.g., Unexamined Japanese Patent Application Publication No. H06 (1994)-258949).
By this method, the charge amount of toner supplied to the toner carrying member can be measured in the actual operation. And based on the measured charge amount, it is possible to control the amount of toner deposited on the toner carrying member to be kept at a constant level.
As described above, to control the formation of the toner layer on the toner carrying member so that stable image quality can be provided, it is necessary to accurately detect the amount of the deposited toner, which fluctuates depending on the printing environment, the total number of printed sheets, and/or the number of continuous printing sheets.
The Unexamined Japanese Patent Application Publication No. 2008-176236 discloses a technique on the optical detecting method for detecting the deposited toner amount of the toner layer formed on the toner carrying member. In this technique, light is applied to the toner layer, and the reflected light is detected by the light receiving element. The amount of toner deposited on the toner layer is obtained from the detection result.
However, this technique is accompanied by the following problem. There are two methods to capture the reflected light, i.e. one method is to capture the scattered light from toner, and the other one is to capture the reflected light from the toner carrying member. However, if the amount of toner deposited on the toner carrying member is excessive, neither method can detect a change in the amount of the deposited toner.
Unexamined Japanese Patent Application Publication No. H06 (1994)-258949 discloses a technique in which the amount of the charged toner supplied to the toner carrying member from the developer carrying member is obtained by analyzing the current flowing through the closed loop circuit including a toner carrying member and a developer carrying member. As the amount of charged toner supplied to the toner carrying member can be measured in the actual operation, it is possible to keep the amount of the toner deposited on the toner carrying member at a constant level.
However, a specific charge of the toner itself depends on the printing environment and/or the total number of printed sheets and the number of continuous printing. Therefore, a desired amount of toner cannot always be obtained even when the control is provided according to the calculated amount of deposition (charge amount of deposited toner). For example, if there is an increase in the specific charge of the toner, the amount of the toner to be deposited on the toner carrying member will be reduced. This will make it difficult to ensure sufficient image density.